1. Field of the Invention
The present invention generally relates to an improved bandgap reference circuit capable of improving the electrical characteristic of power supply rejection ratio (PSRR) and temperature coefficient (TC) thereof.
2. Description of Related Art
For a digital-to-analog converter (DAC), an analog-to-digital converter (ADC) or a regulator, at least a fixed and stable reference voltage is required to the operation thereof. The reference voltage is preferably to be stably regenerated whenever starting up the power supply. An ideal reference voltage is preferably free from influences of process nonconformance, operation temperature change and power source variance.
It is well known that a bandgap reference circuit is suitable for providing a reference voltage. Thus, in a number of electronic systems, a bandgap reference circuit plays an important role since a bandgap reference circuit would vitally affect the stability and accuracy of the system.
Usually, a bandgap reference circuit includes following major components: a current mirror, an operation amplifier, a bandgap current generator and a load.
FIG. 1 is a schematic drawing of a conventional bandgap reference circuit. The bandgap reference circuit includes MOS transistors (metal oxide semiconductor transistor) M11-M13, an operation amplifier OP1, BJTs (bipolar junction transistors) Q11 and Q12, resistors R11 and R12 to constitute a bandgap current generator and a load R13.
The bandgap current generator in FIG. 1 includes two current paths, through which two currents I1A and I1B generated thereby respectively flow and I1A=I1B=I11+I12. The current I11 herein is a Proportional solute Temperature (PTAT) current, while the current I12 is a Complementary solute Temperature (CTAT) current; therefore, the resulting current I1A or I1B of the currents I11+I12 is regarded as a temperature-independent current. In addition, thanks to the operation of a current mirror, I1C=I1A=I1B; thus, I1C is also regarded as a temperature-independent current. Furthermore, because VREF=I1C*R13, the reference voltage VREF generated by the bandgap current generator is regarded as a temperature-independent current as well.
In consideration of the channel-length-modulation effects of the MOS transistors, I1A=I1B≠I1C. The cause of the unidentical relationship herein is that although an effect of virtual ground (V1A=V1B) results in the drain-source voltages of the MOS transistors M11 and M12 are identical to each other; but another node voltage V1C is not necessarily identical to V1A or V1B. As a result, the drain-source voltages of the MOS transistors M11 and M12 are not necessarily identical to the drain-source voltage of the MOS transistor M13, i.e. VDSM11=VDSM12≠VDSM13. Such mismatch of the drain-source voltages is quite sensitive to the power source and the temperature, which would lead to a poor power supply rejection ratio (PSRR) and an unacceptable temperature coefficient (TC).
Based on the above-described situation, it is highly desirable to improve the conventional bandgap reference circuit to overcome the disadvantages of the prior art, i.e. capable of providing a better temperature coefficient and improving the poor PSRR characteristic. Besides, the improved bandgap reference circuit should be designed without specific circuit components and fabricated by standard CMOS (complementary metal oxide semiconductor transistor) processes.